War seems to be part of the human condition. We have records of war going back beyond written records, and there is even evidence that some animals like chimps and ants go to war as well. But why do we do it? Here are ten of the most important theories.

1. The Male Warrior Hypothesis

Formulated by a group of evolutionary psychologists, this hypothesis suggests that men evolved to be violent and warlike in order to secure access to women and other resources. Essentially, forming violent coalitions with fellow men was a mating strategy. The more successful the "war coalition" was, the more successful the men would be in passing along their genes. Often this idea is reduced to the notion that men's sex drives are at the root of war, which is only half the story. In fact, the idea is that men evolved to form war bands with each other to gain access to resources. Having such resources would have made them better able to support families and communities, and thus pass along a genetic predisposition for forming armies.

Another version of this idea is the "demonic male hypothesis," which suggests that the urge to go to war goes back to the last common ancestor between humans and apes. Because chimps exhibit behavior that is warlike — with one band of males attacking another band — evolutionary biologists have suggested that human males inherited the urge to make war from distant evolutionary ancestors that we share with other hominids.

2. War as Predation

Essayist Barbara Ehrenreich spent many years researching the origins of war, and determined that the male warrior hypothesis didn't exactly fit the facts. Instead, she suggests that war grows out of the ancient human fear of predatory animals. When humans were evolving, one of our formative experiences as a species would have been hiding from more skillful predators than Homo sapiens. But once we'd gained the tools necessary to be predators ourselves, we celebrated this accomplishment in "blood rites" of sacrifice. These rites began as hunting rituals, but over time evolved into war rituals with neighboring humans. This theory explains why war doesn't usually feel "natural" to most men, and requires a kind of ritualistic transformation like a religious warrior ritual or Basic Training. War is a learned behavior, and its rituals are a defense against fear of predation.

3. The Persuasive Hawk

In debates over conflicts, there are hawks and doves, with hawks favoring forceful actions to end tensions and doves advocating negotiation. Hawks usually win because of inherent biases we all have. Nobel laureate in economics Daniel Kahneman and government researcher Jonathan Renshon crystallized this idea in a famous article for Foreign Policy, where they explained that, oddly, the Persuasive Hawk Theory is a result of humanity's optimism bias:

Psychological research has shown that a large majority of people believe themselves to be smarter, more attractive, and more talented than average, and they commonly overestimate their future success. People are also prone to an "illusion of control": They consistently exaggerate the amount of control they have over outcomes that are important to them — even when the outcomes are in fact random or determined by other forces.

In other words, we go to war because we mistakenly believe that we are always going to win, because we are the best. A related idea is the "Rubicon Theory," which suggests that when people believe they are already threat they cross a psychological threshold where new biases take over. Instead of proceeding rationally, they become overconfident and engage in riskier behavior — such as starting a war instead of seeking peaceful alternatives.

4. Malthusian Overpopulation

Based on Thomas Malthus' population theories, this idea suggests simply that war is the inevitable result of an expanding population with scarce resources. Stanford economist Ran Ambramitzky explains this idea quite simply in a paper. The human population increases at a geometric rate, faster than the food supply. Voluntary "preventative checks" try to keep population growth down, such as when people make rational decisions about the number of kids they are going to have based on their income, etc. When these checks fail, "positive checks," including war, famine and diseases, reduce the population and balance it with resources. Malthus believed that as long as humanity didn't come up with decent preventative checks, the positive check of war would ensure that population didn't outstrip food supply.

This idea overlaps a bit with the "ecological imbalance" theory of war, in which "conflict flash points" are the result of ecological stress from humans exploiting too many resources from the land. When resources run out, conflicts arise.

5. Youth Bulge

A popular theory right now, this idea suggests that violence and wars are the result of a large population of men with a lack of peaceful employment opportunities. The excess youth will be drawn to fighting and be killed, reducing the population.

6. Groupthink

Groupthink theory explains that during a crisis, groups — no matter how smart or well-informed — will suppress dissenting opinions because of the pressure to agree on a plan of action, leading them to make terrible decisions. This is in some sense a more policy-oriented version of the male warrior theory crossed with the persuasive hawk. The idea is that, when threatened, people naturally form bands of "us" vs. "them," and then make risky decisions in order to maintain their sense of superior group identity. Political scientists have recently applied the theory to the Iraq war.

7. Bargaining Model

Perhaps, say some social scientists, war isn't a deep-seated urge that or emotional reaction that comes from our evolution. Maybe it's just a form of political maneuvering that we've developed along with civilization. Seen in this light, war is just an extreme version of bargaining, where two groups try to resolve disputes over everything from allocation of resources to social justice. Writes scholar Dan Reiter:

Critically, the bargaining model does not see war as the breakdown of diplomacy but rather as a continuation of bargaining, as negotiations occur during war, and war ends when a deal is struck.

This model is helpful for international relations, because it suggests that every war is a negotiation and resolution waiting to happen.

8. Terror Management

The theory suggests that humans form cultural groups such as tribes and nations because they need to believe in something that will live on after they die. We all fear our own mortality, but our cultures give us beliefs and rituals that buffer us from that fear. Problems arise when these beliefs are threatened. Terror management theory suggests that for many people, an attack on their nation or group arouses their basic fear of death. You can see traces of the Rubicon theory here, where threats to the group cause people to cross a threshold where they are willing to make violent decisions that they would never make in everyday life. Terror management theory holds that crossing this threshold makes people willing to die to preserve their culture — because, after all, it is only their culture that can live on after them.

9. The Aggressive Drive

Aggression is a fighting instinct that helps individuals and species survive. In animals, there are innate inhibitions against killing others of the same species, such as the display of submissive gestures. But it's different for humans: weapons and communal aggression ("militant enthusiasm") escalate our ability to defend ourselves, but also to inflict violence on other groups. The inevitable expression of human aggression is war. This idea suggests that war is specific to humanity, as a result of our advanced tools and social organization.

9. The Aggressive Drive

Aggression is a fighting instinct that helps individuals and species survive. In animals, there are innate inhibitions against killing others of the same species, such as the display of submissive gestures. But it's different for humans: weapons and communal aggression ("militant enthusiasm") escalate our ability to defend ourselves, but also to inflict violence on other groups. The inevitable expression of human aggression is war. This idea suggests that war is specific to humanity, as a result of our advanced tools and social organization.

10. War Is Learned (And Can Be Unlearned)

First proposed by anthropologist Margaret Mead in the early twentieth century, this hypothesis suggests that war is not the inevitable consequence of our competitive, aggressive nature. Rather, it is a social invention that can be unlearned. This actually dovetails with the "aggressive drive" theory, which suggests that humans may be aggressive like other animals — but our social organization is what leads to war. It is also a sharp rebuke to the evolutionary psychology idea of warrior men, and to the neo-Malthusian notion that war is inevitable when our population grows. Given that war is a social response to our environment and to each other, it makes sense that the solution to war would be social as well. We can learn peace instead of learning war — and we don't have to change our genomes to do it.

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O Almighty Lord God, who neither slumberest nor sleepest; Protect and assist, we beseech thee, all those who at home or abroad, by land, by sea, or in the air, are serving this country, that they, being armed with thy defence, may be preserved evermore in all perils; and being filled with wisdom and girded with strength, may do their duty to thy honour and glory; through Jesus Christ our Lord. Amen.

O Almighty Lord God, who neither slumberest nor sleepest; Protect and assist, we beseech thee, all those who at home or abroad, by land, by sea, or in the air, are serving this country, that they, being armed with thy defence, may be preserved evermore in all perils; and being filled with wisdom and girded with strength, may do their duty to thy honour and glory; through Jesus Christ our Lord. Amen.